Update Merge request management view

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What does this MR do and why?

This MR updates the Request Review section on the merge request management page based on discussion : #219 (comment 2766847592)

Changes

• Replaced modal-based review request with a unified form UI using checkboxes and textarea.

• Added reactive reviewer state with reviewerSelections and computed selectedReviewers.

• Defaulted coach selection when no other reviewers exist.

• Added a single “Request review” button.

Screenshots or screen recordings

Before	After

When a MR has an existing reviewer:

• Default checkboxes to existing reviewers
• Default unchecked on random available coach
• GitLab Duo AI check by default

Screen_Recording_2025-10-13_at_7.42.34_PM

When a MR doesn't have existing reviewers:

• Default checkbox to random available coach
• GitLab Duo AI check by default

Validation steps

/cc @leetickett-gitlab

Closes #219